1. Field of the Invention
The present invention relates to technology for preventing periodic signal components (in-band tones) from being generated from a delta-sigma modulator of a delta-sigma Analog-to-Digital Converter (ADC) and a multi-bit Digital-to-Analog Converter (DAC) used in a delta-sigma DAC, and more particularly, to a dynamic element-matching method, a multi-bit DAC using the method and a delta-sigma modulator and delta-sigma DAC including the multi-bit DAC.
This work was supported by the IT R&D program of MIC/IITA [2006-S-006-02, Components/Module technology for Ubiquitous Terminals].
2. Discussion of Related Art
At an Input/Output (I/O) end or transceiver end of a specific application operating at low frequency and requiring high resolution, signal conversion is performed using a delta-sigma ADC and a delta-sigma DAC.
FIGS. 1A and 1B are block diagrams of a conventional delta-sigma ADC 100 and a conventional delta-sigma DAC 200, respectively. FIG. 1C schematically illustrates a structure of a conventional multi-bit DAC.
Referring to FIG. 1A, the delta-sigma ADC 100 comprises a delta-sigma modulator 110 and a Low-Pass Filter (LPF) 130. The delta-sigma modulator 110 comprises an adder 111, an integrator 112, an ADC 113 and a DAC 114. In a multi-bit configuration, the ADC 113 and the DAC 114 are configured for multiple bits.
Referring to FIG. 1B, the delta-sigma DAC 200 comprises a delta-sigma modulator 210, a DAC 220 and an LPF 230. In a multi-bit configuration, the delta-sigma modulator 210 and the DAC 220 are configured for multiple bits.
When the DAC 114 shown in FIG. 1A and the DAC 220 shown in FIG. 1B are configured for multiple bits, respective bits of a digital input code switch analog unit elements, for example, capacitors corresponding thereto and are converted into analog signals, and the analog signals are added together and output, as illustrated in FIG. 1C. In this structure, the non-linear characteristic of digital-analog conversion is caused by a mismatch between unit elements.
To solve the problem of a mismatch between unit elements, research has been actively conducted on Dynamic Element Matching (DEM) such as random averaging, Clocked Averaging (CLA), Individual Level Averaging (ILA), Data Weighted Averaging (DWA), and so on. For example, use of a dynamic element-matching technique has been disclosed in U.S. Pat. No. 5,990,819 “D/A converter and Delta-Sigma D/A converter”, and so on.
Using the dynamic element-matching technique, it is possible to select unit elements in random sequence with every operation of a DAC and convert a mismatch between elements into white noise.
In particular, when a rotation algorithm such as a DWA technique, which selects unit elements for an input digital code, is used, mismatches between unit elements are averaged, such that noise caused by an element mismatch can be shaped within a signal band.
The basic technology of the DWA has been disclosed in a periodical “Rex T. Baird, Terry S. Fiez, Linearity Enhancement of Multibit ΔΣA/D and D/A Converters Using Data Weighted Averaging, IEEE Transaction on Circuits and Systems-II: Analog and Digital Signal Processing, Vol. 42, No. 12, December 1995”, and so on.
According to a conventional DWA algorithm, unit elements are selected for an input digital signal in sequence or in a simply changed sequence. Therefore, when a specific signal is repeatedly input, periodical signal components, i.e., in-band tones, are generated.
FIG. 2 illustrates a method of selecting a unit element according to the conventional DWA algorithm and a mismatch error according to the method. In FIG. 2, the mismatch error is when a 3-bit input signal is applied to 8 unit elements.
As illustrated in FIG. 2, according to the conventional DWA algorithm, the same unit element is selected again for a ninth digital input signal, and thus a DAC mismatch error is repeated every eight cycles. Due to the periodicity of unit element use, a tone is generated at a specific input frequency.
The generation of tones is not preferred because the tones reduce a dynamic range, modulate nose outside a preferable signal band, and interfere with signals within the preferable signal band. Even when the tones exist below the minimum noise, they can be heard in an audio converter. In particular, the problem of the tones becomes prominent as the amount of input digital data decreases.
Therefore, modified DWA techniques for reducing tone generation are being widely researched, and as a result of the research, techniques, such as bi-directional DWA, rotated DWA and incremental DWA, have been proposed in the art. However, the techniques have a problem in that a signal-to-noise ratio increases due to an increase in noise within a signal band, or tones are still generated.